There are an abundance of organic chemical reactions between functionally reactive organic compounds to make condensation or addition products in which it would be desired to suppress the rate of reaction, even blocking the occurrance of the reaction, until select conditions are achieved. When that occurs, it would be desirable to unsuppress or unblock the reaction and produce, in the typical manner, the desired or expected reaction product. Included in these organic chemical reactions are the coreaction of amino groups with other functional groups via condensation or addition (e.g., Michaels addition).
Oftentimes, the amino containing reactant contains another functional group that has the potential of reacting with either the amino functionality and/or the functional group of a co-reactant. In those cases, one may desire to block the reaction of the amino group in favor of the other group on that organic compound.
There are many polymeric reaction systems, typically of the thermosettable resin type, that depend on the blending of organic monomers, oligomers or polymers that possess interreactive (typically complementary) functionality. In most cases, it would be desirable to suppress or block the reaction of such systems in order to avoid premature reactions that result in wastage of the resin components by having a reaction occur before the reaction system has been shaped or applied for production of the ultimate end product. This is often called extending the pot-life of the reaction system. There are applications where some level of reaction is developed followed by suppression of the reaction to prevent full cure. For example, there are situations where a thermosettable resin is applied to a fibrous mass of strands, bundles or staple forms of fiber to make a fiber reinforced plastic ("FRP") and it would be desirable to allow such systems to partially cure to the B-stage only. In those cases, it would be desirable to arrest full cure until the system is ready to be applied to a substrate, mold, etc., and then be subjected to final cure, called the C-stage. Oftentimes, those applications would cure to premature unshapable conditions and the applications have to be discarded as waste. This can occur with SMC (i.e., sheet molding compounds) and prepregs.
There are resin systems from multicomponents that are mixed immediately prior to use. One such .system is called reaction injection molding ("RIM"). In RIM systems, complementary reactive components are premixed immediately prior to being injected into the mold. Premature reaction is avoided by completing the mixing and injection into the mold before the resin components interreact to a condition that renders the system incapable of being effectively injected into the mold. Because RIM is a relatively high speed process, the complementary reactive components possess a very short pot-life. The mixing step entails the use of continuous mixers just before entry into the mold and the reactants are separately fed to the mixer to avoid premature reaction. It would be desirable to be able to premix these reactants without premature reaction in other type of mixing means, even store the mixture, and then have the ability to inject the reactants into a mold. It would be desirable to be able to suppress the reaction until the reactants have adequately filled the mold.
There are many polymeric systems used to effect coating or adhesive application to a substrate where it is desirable to operate at the minimum viscosity. For example, coatings have better flow control at lower viscosities. Adhesives better penetrate a substrate such as wood when applied at lower viscosities. However, controlling their viscosities is dependent upon the ability to suppress interreactions between the resin forming components. It would thus be desirable to be able to suppress or block such reactions until the application has been effected.
This invention provides the ability to selectively suppress such chemical reaction by incorporating a supercritical fluid, especially carbon dioxide maintained under supercritical fluid (SCF) conditions into the chemical reaction system. The advantage of reaction suppression is one of a number of advantages provided by this invention.
Because of environmental concern in recent years, there is an interest in finding ways of reducing pollution resulting from painting and finishing operations. For this reason there has been a great deal of emphasis placed on the development of new coating technologies which diminish the emission of organic solvent vapors. A number of technologies have emerged to meet most but not all of the performance and application requirements, and at the same time meet emission requirements and regulations. They are: (a) powder coatings, (b) water-borne dispersions, (c) water-borne solutions, (d) non-aqueous dispersions, and (e) high solids compositions. Each technology has been employed in certain applications and each has found a niche in a particular industry. However, at the present time, none has provided the performance and application properties that was hoped for initially.
Powder application, for example, while providing ultra-low emission of organic vapors, is characterized by poor gloss or good gloss with heavy orange peel, poor definition of image gloss with heavy orange peel, and poor film uniformity. Pigmentation incorporation is often a problem that at times requires milling and extrusion of the polymer-pigment composite mixture followed by cryogenic grinding. In addition, changing colors of the coatings line often requires a complete cleaning because of dust contamination of the application equipment and finishing area.
Water-borne coatings cannot be applied under conditions of high relative humidity without serious coating defects. These defects result from the fact that under conditions of high humidity, water evaporates more slowly than the organic co-solvents of the coalescing aid. As might be expected, in the case aqueous dispersions, the loss of the organic cosolvent/coalescing aid interferes with film formation resulting in poor gloss, poor uniformity and pin holes. Additionally, water-borne coatings are not as resistant to corrosive environments as are the more conventional solvent-borne coatings.
Coatings applied from organic solvents at high solids avoid many of the pitfalls of powder and water-borne coatings. In these systems, the molecular weight of the polymer is low and reactive functionality is relied upon to further polymerization and crosslinking after the coating has been applied. In reality, there is a limit in the ability of this technology to meet the performance requirements of a commercial coating operation. Present high solids systems are difficult to apply to vertical surfaces without running and sagging of the coating. Often they are prone to cratering and pin-holing. If they have good reactivity they have poor shelf and pot-life; if they have shelf stability then often they cure and/or crosslink slowly or require high temperature and energy to convert.
It would be desirable to suppress the reactivity of such coating systems possessing reactive functionality, so that they possess the desired amount of functionality so as to react quickly enough when coated onto the substrate yet not prematurely react so as to adversely affect coating quality.
The use of supercritical fluids as a transport medium for the manufacture of surface coatings is well known. German patent application 28 53 066 describes the use of a gas in the supercritical state as the fluid medium containing the solid or liquid coating substance in the dissolved form. In particular, the application addresses the coating of porous bodies with a protectant or a reactive or nonreactive decorative finish by immersion of the porous body in the supercritical fluid coupled with a pressure drop to effect the coating. The most significant porous bodies are porous catalysts. The applicant characterizes fabrics as porous bodies.
Smith, U.S. Pat. No. 4,582,731, patented Apr. 15, 1986, and U.S. Pat. No. 4,734,451, patented Mar. 29, 1988, describes forming a supercritical solution which includes a supercritical fluid solvent and a dissolved solute of a solid material and spraying the solution to produce a "molecular spray." A "molecular spray" is defined as a spray "of individual molecules (atoms) or very small clusters of the solute." The Smith patents are directed to producing free films and powders. The films are used as surface coatings.
U.S. patent application Ser. No. 133,068 and U.S. Pat. No. 4,923,720, to Lee et al., disclose a process and apparatus for the liquid spray application of coatings to a substrate and minimize the use of environmentally undesirable organic diluents. The process of the application involves:
(1) forming a liquid mixture in a closed system, said liquid mixture comprising: PA1 (2) spraying said liquid mixture onto a substrate to form a liquid coating thereon. PA1 (1) means for supplying at least one polymeric compound capable of forming a continuous, adherent coating; PA1 (2) means for supplying at least one active organic solvent; PA1 (3) means for supplying supercritical carbon dioxide fluid; PA1 (4) means for forming a liquid mixture of components supplied from (1)-(3); and PA1 (5) means for spraying said liquid mixture onto a substrate. PA1 (1) forming a liquid mixture in a closed system, said liquid mixture comprising: PA1 (2) spraying said liquid mixture onto a substrate to form a liquid coating thereon by passing the mixture under pressure through an orifice into the environment of the substrate to form a liquid spray. PA1 (1) forming a liquid mixture in a closed system, said liquid mixture comprising: PA1 (2) spraying said liquid mixture onto a substrate to form a liquid coating thereon by passing the mixture under pressure through an orifice into the environment of the substrate to form a liquid spray; and PA1 (3) electrically charging said liquid spray by a high electrical voltage relative to the substrate and electric current.
(a) at least one polymeric compound capable of forming a coating on a substrate; and PA2 (b) at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture of (a) and (b) to a point suitable for spray applications; and PA2 (a) at least one polymeric component capable of forming a coating on a substrate; and PA2 (b) a solvent component containing at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture to a point suitable for spray application; and PA2 (a) at least one polymeric component capable of forming a coating on a substrate; and PA2 (b) a solvent component containing at least one supercritical fluid, in at least an mount which when added to (a) is sufficient to render the viscosity of said mixture to a point suitable for spray application;
They are also directed to a liquid spray process in which at least one active organic solvent (c) is admixed with (a) and (b) above prior to the liquid spray application of the resulting mixture to a substrate. The preferred supercritical fluid is supercritical carbon dioxide. The process employs an apparatus in which the mixture of the components of the liquid spray mixture can be blended and sprayed onto an appropriate substrate. The apparatus contains
The apparatus may also provide for (6) means for heating any of said components and/or said liquid mixture of components. U.S. patent application Ser. No. 133,068 and the patent demonstrate the use of supercritical fluids, such as supercritical carbon dioxide fluid, as diluents in highly viscous organic solvent borne and/or highly viscous non-aqueous dispersions coatings compositions to dilute the compositions to application viscosity required for liquid spray techniques. They further demonstrate that the method is generally applicable to all organic solvent borne coatings systems.
Copending U.S. application Ser. No. 218,910 is directed to a liquid coatings application process and apparatus in which supercritical fluids, such as supercritical carbon dioxide fluid, are used to reduce to application consistency viscous coatings compositions to allow for their application as liquid sprays. The coatings compositions are sprayed by passing the composition under pressure through an orifice into the environment of the substrate.
In particular, the process of U.S. application Ser. No. 218,910 for liquid spray application of coatings to a substrate comprises:
U.S. application Ser. No. 218,895 is directed to a process and apparatus for coating substrates by a liquid spray in which 1) supercritical fluid, such as supercritical carbon dioxide fluid, is used as a viscosity reduction diluent for coating formulations, 2) the mixture of supercritical fluid and coating formulation is passed under pressure through an orifice into the environment of the substrate to form the liquid spray, and 3) the liquid spray is electrically charged by a high electrical voltage relative to the substrate.
In particular, the process of U.S. application Ser. No. 218,895 for electrostatic liquid spray application of coatings to a substrate comprises:
The above technologies amply demonstrate the applicability of supercritical fluids as a carrier and viscosity reducer for transporting a variety of coating materials and effectively spraying them onto a coatable surface while reducing the amount of volatile organic compounds (VOCs) that are required for the application.